1. Field of the Invention
This invention relates broadly to wireless communication systems and, more particularly, to demodulation and decoding of a backscatter radio frequency signal in a radio frequency identification system.
2. State of the Art
Radio Frequency Identification (RFID) systems are used for identification and/or tracking of equipment, inventory, or living things. RFID systems are radio communication systems that communicate between a radio transceiver, called an Interrogator, and a number of inexpensive devices called Tags. The objectives of RFID systems are to design a reliable and secure architecture, and to minimize the total cost of the Interrogator and the Tags, while meeting the system performance requirements.
In RFID systems, the Interrogator communicates to the Tags using modulated radio signals, and the Tags respond with modulated radio signals. For downlink communication from the Interrogator to a Tag, the Interrogator transmits a modulated radio signal that encodes the Interrogator's message. The Tag receives the modulated radio signal and demodulates and decodes the Interrogator's message therefrom. For uplink communication from a Tag to the Interrogator, the Interrogator transmits a continuous-wave (CW) carrier signal. The CW carrier signal can be a frequency-hopping spread-spectrum (FHSS) carrier signal as is well known, thereby enhancing the system's ability to operate in a multipath environment. The Tag modulates the CW carrier signal using modulated backscattering operations whereby the antenna is electrically switched from being an absorber of RF radiation to being a reflector of RF radiation, thereby encoding the Tag's information onto the CW carrier signal. The Interrogator receives the incoming modulated CW carrier signal and demodulates and decodes the Tag's information message therefrom. The uplink and downlink communication occurs in a half-duplex manner such that a Tag will not perform communication while it is waiting for communication from an Interrogator and also will not interpret communication from the Interrogator while it is communicating. The Tag can be a passive-type tag that obtains its operating energy by rectifying the RF energy transmitted by the Interrogator and received at the Tag's antenna. Alternatively, the Tag can be a semi-passive tag (sometimes referred to as semi-active tag) that is equipped with at least one battery to provide operating energy to the Tag.
As described above, the Interrogator operates to receive the reflected and modulated CW carrier signal and demodulate and decode the Tag information message encoded therein. Typically, such functionality is accomplished by homodyne detection wherein the received signal is amplified with a low noise amplifier whose output is mixed by a quadrature mixer that uses the same RF signal source as the transmit functionality. The in-phase (I) and quadrature (Q) components output from the quadrature phase mixer are filtered and processed by a data recovery circuit. The data recovery circuit can be realized in many different ways including both analog, digital and hybrid analog/digital implementations. Typically, these implementations perform integrate and dump operations whereby the signal energy of the I component and/or Q component is (are) accumulated during a symbol period. The accumulated value(s) is (are) supplied to a symbol decision comparator that produces the demodulated data stream. An example of such a receiver implementation is described in U.S. Pat. No. 6,456,668 to MacLellan et al.
Disadvantageously, the integrate and dump methodology of the prior art receiver designs has poor performance because it provides limited knowledge of the energy of the signal as well as the noise process of the communication channel. These limitations reduce the signal to noise ratio of the receiver subsystem, which results in increased signal power at the Tag (or decreased read range of the system) in order to maintain a prescribed bit error rate. The increased signal power at the Tag is typically realized by a larger Tag antenna, which increases the size and costs of the Tag.
Therefore, there remains a need in the art for RFID Interrogators and RFID systems based thereon that provide improved receiver performance (i.e., an improved signal to noise ratio) which allows for reduced signal power at the Tag (or a larger read range of the system) while maintaining a prescribed bit error rate. Such improved receiver performance advantageously will not require an increase in the size and cost of the Tag.